RunRyder RC
WATCH
 1 page 755 views POST REPLY
HomeAircraftHelicopterBeginners Corner › CoG on 450 fb, fbl ???
12-10-2012 05:07 AM  5 years agoPost 1
icanfly

rrElite Veteran

ontario

My Posts: All  Forum  Topic

Not long ago I slung a GoPro under my 450. It balanced and flew better than without the 90 grams of camera between the skids, like a plumb bob loading the disc a bit more.

Off goes the GoPro and the 450 is slippery again. Cog is centered on the fb, heavier blades (35gr) keep it stable.

When I was doing a little handling last week I noticed the heli tips over. I had held it by the newly cleansed boom (relocated the servo to under the tail block) just behind the tail block and it wanted to roll over. That would mean my fb/blade assembly is heavier than everything below the boom.

You would think that there might not be any effect of the fb assembly weight on stablity. Maybe it does since the boom is putting a cantilever force on the heli. Half the battery is mounted a little above the roll axis too.

It comes to this, I firmly believe that a heli will be much more stable if it is balanced on all axis from nose to tail (pitch), side to side (roll), and north to south poles, or, on the main shaft axis (yaw).

If your heli is top heavy along the roll axis along the boom, you might try balancing it with weights. I can only guess a single 20gram weight might correct it, maybe more, mounted on the base plate below the main shaft.

Maybe no one notices the small things but try it out. If it helps stabilize a fb 450 just that bit more.

Haven't checked my fbl 450 yet but will get around to it one day.

thought I'd share this, cheers.

PM  EMAIL  GALLERY  Attn:RR  Quote
12-10-2012 06:13 AM  5 years agoPost 2
Dood

rrProfessor

Wescanson

My Posts: All  Forum  Topic

I know you just spent all that time writing that big long post, but I'm going to negate everything you just said and say this:

We WANT it top heavy! The closer the CG is to the rotor head the better. The idea is to minimize the mass moment of inertia on the roll axis and also the pitch axis.

You may have noticed a current trend of lowered rotor heads, bring the rotor down towards that CoG.

Adding weight to a helicopter to balance it is also frowned upon.

  ▲
▲ ▲

PM  EMAIL  GALLERY  Attn:RR  Quote
12-10-2012 02:07 PM  5 years agoPost 3
icanfly

rrElite Veteran

ontario

My Posts: All  Forum  Topic

Adding weight to a helicopter to balance it is also frowned upon.
I understand that too.

Problem is this, the tail rotor is below the main rotor and is pushing it off the horizontal axis with leverage. The fulcrum, or point of axis between tail and main rotors, is below the roll cg.

Everyone's seen a frizbee that flips over if it's tilted even slightly. Throw it right hand and it will revolve in the same direction as a heli rotor.

I guess the only solution to the rotor tilt is to bring the tail rotor in axis with the main rotor, otherwise it'll always be a matter of adjusting to an added variable, leverage.

It's not such a big post.

My gp780 manual says the gyro should be mounted perfectly horizontal so not to be affected by roll or pitch axis. Trouble is the little bugger is so sensitive it'll pick up inputs from them. PROBLEM lies in when the heli is in the air and exhibiting a roll to the right.

I never thought a tail gyro could affect flight stability but it had when I changed from a gy240 rate gyro to the 780.

Mastering a heli in the basic rc 'rs standard configuration is difficult when there are many unseen forces acting upon it. They are hard to fly until you figure it all out and compensate on the fly.

My first handfull of smash downs were a result of these adjustments and imbalances, or lack of.

The 780 is holding it pretty level and steady even in wind, I'm impressed.

Did a quick test a few minutes ago and it was coming at me disc flat toward my face but I feared not and gave it full forward for an immediate correction. WOW, feels good to have much better control with the Align gyro.

PM  EMAIL  GALLERY  Attn:RR  Quote
12-12-2012 12:27 AM  5 years agoPost 4
icanfly

rrElite Veteran

ontario

My Posts: All  Forum  Topic

This topic has been mostly about cog. As important is chassis flex/stiffness.

I had to remove my 780 due to torsional flex between the main bearings and the tail block. The cf frame is weak and you can twist the main shaft bearing blocks and the tail block enough that an ultra sensitive gyro will pick it up and try to compensate, alas the dreaded tail waggle.

Next little job on improving an old 450 is an aluminum plate bolted between the tail block and main bearing block screwed on both frame sides to stiffen it up. It's one place that can do without flex.

PM  EMAIL  GALLERY  Attn:RR  Quote
12-15-2012 05:34 PM  5 years agoPost 5
RICHW

rrVeteran

Cupertino, CA

My Posts: All  Forum  Topic

The American rocket pioneer Goddard designed his first rocket with the engine high above the fuel tanks on a steel frame, with the belief that the lowered center of gravity would help keep the rocket pointing straight up as it flew. It didn't. It is surprising that he could not see that the rocket is going to go in whatever direction the motor is pointing away from. This isn't too different than a heli arrangement. Gravity will not play a part in stabilizing a heli, only help return it to Earth. I concur that the CG should be as close to the head as possible for snappy manuevers.

PM  EMAIL  Attn:RR  Quote
12-16-2012 11:59 AM  5 years agoPost 6
icanfly

rrElite Veteran

ontario

My Posts: All  Forum  Topic

It's easy for a beginner to blame them self for not "getting" the controls of a heli, and give up.

Beginners don't understand that a twin blade heli 450 is inherently unstable due to off axis tilt because the tail rotor is below the axis of main disc rotation. The heli will slide of to the right, nose out, more easily then to the left due to this imbalance.

Setting up a heli with a little extra weight, servos, on the left side of the heli will pull down the left side of the disc ever so little but a better flight cog is the result.

Controlling a heli is easier when the rotor disc is perfectly horizontal. 3 axis gyros make adjustments to attain this. If you fly with flybar you are going to work harder to maintain total control at all times.

On a heli with the tail rotor up high, main and tail axis on equal plane, a heli will be much easier to fly if it is not fbl.

PM  EMAIL  GALLERY  Attn:RR  Quote
WATCH
 1 page 755 views POST REPLY
HomeAircraftHelicopterBeginners Corner › CoG on 450 fb, fbl ???
 Print TOPIC  Make Suggestion 

 2  Topic Subscribe

Tuesday, July 17 - 10:48 am - Copyright © 2000-2018 RunRyder   EMAILEnable Cookies

Login Here
 New Subscriptions 
 Buddies Online